Paper filter media have generally been used as disposable vacuum cleaner bags. The paper filters are comparatively inexpensive but suffer from being ineffective in the removal of extremely fine dust and dirt particles. More problematic with such filter bags is that they tend to become plugged with the trapped dirt.
The porosity of the paper filter media allows passage of fine dust through the bag wall thus permitting the filter to be at least partially filled with particulates before the air pressure drop across the bag wall increases to an unacceptable level. Reduction of the porosity of the media can improve filtration performance of the media but the effect is to increase the air pressure drop across the media. Additionally, reduced porosity of the filter media enables dirt particles to accumulate on the media surface at a faster rate than for a more porous filter, thereby causing a more rapid rate of increase in the pressure drop across the media. This phenomenon shortens the service life of the filter.
The recognition of the adverse effects of inhaled particles, especially particles in the sub-10 micron range, has led to the development of filter materials to remove these minute particles from air streams. These filters are commonly designated as HEPA (High Efficiency Particulate Air) and ULPA (Ultra Efficiency Particulate Air) filters.
The use of nonwoven microfibrous mats as the filter media for vacuum cleaner filters is known as an alternative to paper filter media. In general, microfibrous mats have higher particle capture efficiencies with comparable or lower pressure drops than paper media. However, microfibrous mats having the required air permeability are typically thicker than paper filters and tend to be significantly weaker than paper media. To circumvent this problem, a carrier fabric has been used to help facilitate handling the nonwoven. As a result, the composite layers have been thicker than standard paper filter media and do not have the handling characteristics of paper media so that they could be converted into vacuum cleaner filter bags with the same equipment used with the paper media.
Conventional filter media are substantially lacking in electrostatic charge and rely upon impingement, impaction and diffusion for filter performance. Electret filter materials offer improved filtering performance over conventional filter materials. The presence of oriented dipoles in the electret filter media is believed to enhance filter performance by allowing the filter media to attract and retain charged and uncharged particles to be filtered.
Electret filter materials are made by a variety of known techniques. One technique for manufacturing electret filter media involves extruding a polymer, typically having a high melt flow index, through a die having a linear array of orifices. An air knife is used to attenuate the extruded polymer fibers by a ratio of about 300:1. The attenuated fibers, having diameters of about one to ten micrometers, are collected on a rotating drum or moving belt using a moderate vacuum. The fiber web is then treated to impart on the fiber web charge pairs or dipoles. The charge pairs or dipoles can be imparted to the fiber, for example, using AC and/or DC corona discharge.
There is thus a need for vacuum filter bags which exhibit the advantageous properties of electret filter media while having improved charge stability and are able to maintain acceptable filter performance over time.
The present invention circumvents the problems described above by providing a multicomponent composite filter media that includes a support layer, at least one layer of a melt blown electret polymer fiber web having a melt processable fatty acid amide present within the web and, optionally, a second support layer. In a preferred embodiment, the composite includes one, two, three or more melt blown electret polymer fiber webs, preferably at least four layers of melt blown electret polymer fibers webs, having the melt processable fatty acid amide present within the web. Generally, all layers are composed of a similar resin, e.g., all layers are prepared from polypropylene. In a preferred embodiment, each layer is laminated to the other layers by ultrasonic welding, ultrasonic bonding patterns which can be edge welded, full width, partial width or combinations thereof, adhesives, thermal bonding, calendering (pressure), or by other methods known to those having ordinary skill in the art.
The invention also provides composite filter media which have enhanced filtration performance characteristics. In particular, the present invention provides charge stabilized electret filter media that is useful for vacuum bags. The bags are both reusable and disposable.
The invention provides composite filter media which includes a melt blown electret polymer fiber web having a melt processable fatty acid amide present within the web. Typically, the melt processable fatty acid amide is present at a concentration of about 0.5% to about 11% by weight, generally between about 1% to 8% by weight, preferably about 1% by weight. Suitable melt processable fatty acid amides, e.g., nonionic fatty acid amides, include stearamides or ethylene bis-stearamides. An exemplary commercially available fatty acid amide is ACRAWAX(copyright)C. In one embodiment the filter media is annealed.
The electret melt blown polymer fiber web layer(s) can be made from a variety of polymeric materials, including polypropylene, polyester, polyamide, polyvinyl chloride, polymethylmethacrylate, and polyethylene. Polypropylene is among the more preferred polymeric materials. Typically, the polymer fibers that form the web have a diameter in the range of about 1 to 15 micrometers, preferably about 3 micrometers, and the weight of the melt blown polymer fiber web is in the range of about 10 to about 100 g/m2, preferably about 35 gm2.
The composite electret filter media of this invention is characterized by improved filtration performance and enhanced charge stability of the electret polymer web layer(s). In particular, the filter media is able to provide desirable filtration properties, as indicated by alpha value, despite continued filtration challenge.
In another aspect the invention relates to a method of manufacturing a composite filter media by melt blowing onto a collecting belt, a first polymer resin having a charge stabilizing fatty acid amide incorporated therein. This produces a first web of melt blown polymer fibers in which the charge stabilizing amide is present at a concentration from about 0.5% to about 11% by weight. The first melt blown web is treated to form substantially permanent charge pairs or dipoles in the first melt blown polymer web, yielding an electret material.
A second melt blown web of polymer fibers having a charge stabilizing fatty acid amide incorporated therein is melt blown onto the first melt blown electret layer. Again, the charge stabilizing amide is present in the polymer resin at a concentration from about 0.5% to about 11% by weight. The second melt blown web is then treated to form substantially permanent charge pairs or dipoles in the second melt blown polymer web, yielding an electret material.
Collectively, the first and second melt blown webs can be termed the composite first component.
A third melt blown web of polymer fibers having a charge stabilizing fatty acid amide incorporated therein is collected on a collecting belt having. This produces a third web of melt blown polymer fibers in which the charge stabilizing amide is present at a concentration from about 0.5% to about 11% by weight. The third melt blown web is treated to form substantially permanent charge pairs or dipoles in the first melt blown polymer web, yielding an electret material.
Optionally, a fourth melt blown web of polymer fibers having a charge stabilizing fatty acid amide incorporated therein is melt blown onto the third melt blown electret layer. Again, the charge stabilizing amide is present in the polymer resin at a concentration from about 0.5% to about 11% by weight. The fourth melt blown web is then treated to form substantially permanent charge pairs or dipoles in the second melt blown polymer web, yielding an electret material.
The third melt blown layer and, optionally, any fourth or subsequent melt blown layers can collectively be termed the composite second component.
The first composite component and the second composite component are then contacted, e.g., laminated, to each other to form the final composite. This can be done by placing one of the melt blown webs of the first composite adjacent one of the melt blown webs of the second composite component and then laminating the two composite components. Optionally, an outermost support layer(s) can be contacted to one of the outermost melt blown layers to provide additional strength to the composite.
The present invention provides the advantage that the melt blown layers are not entangled within or to a substrate layer, e.g., a spunbond material or a support layer, upon which they are formed. Consequently, the melt blown fibers are not coextensively deposited directly on nor do they adhere to a support layer, such as a nonwoven fabric or a spunbond material. This unique process of successive layering of melt blown fiber webs onto each other provides the advantage of control of each layer""s weight basis, and hence, the ability to control the air permeability of each layer and to provide a unique composite. Additionally, the entanglement of fibers between each adjacent melt blown layer provides strength to the overall composite. The enmeshed composite produced by the method of the invention provides a product that is more dense than those presently available. Further, the resulting composite is still chargeable and allows for greater flexibility in ultrasonic bonding, without any burn through.
The permanent dipoles can be imparted to the web by a variety of techniques including AC corona or DC corona discharge and combinations thereof. In an embodiment the manufacturing process can be modified by heat treating the polymer web, which has incorporated therein a melt processable fatty acid amide additive, after charging the material by AC and/or DC corona discharge.
The filter composites of the present invention can be used in a variety of filtration applications in which a relatively stable alpha value is desired. Among the uses for such filter media are industrial face masks, ASHRAE filters, HEPA filters, e.g., HEPA vacuum filter bags, and ULPA filters.
Other advantages of the invention will be readily apparent to one having ordinary skill in the art upon reading the following description.
All percentages by weight identified herein are based on the total weight of each layer of the web unless otherwise indicated.